1. Field of the Invention
The present invention relates to improvements in or relating to flow metering, in particular to a new flow meter and associated method of flow metering for the determination of density, volume and mass flow rates for a single phase flow.
2. Description of Related Art
The metering of fluid flows is a vital part of many engineering processes' control and is therefore directly related to safety and efficiency. In the case of hydrocarbon production wells the fluid flow meter is recording the rate of production and therefore directly recording the monetary flow from the well in question. Clearly, it is vital for industry to have as accurate and reliable a flow meter as possible for many applications. The reality is of course that no meter is ideal for every application and industry is always searching for meters that are cheaper, more reliable, and accurate.
One important class of flow meters are those that are suitable for obtaining a mass or a volume flow rate as a function of density, an example of which is the differential pressure (DP ) flow meter. In the basic sense, a DP flow meter combines Bernoulli's theorem (i.e. the conservation of energy of a fluid flow) and the conservation of mass of a fluid flow between two points in a flow, and the difference in pressure between these two points is measured so that a mass or volume flow rate can be expressed as a function of this differential pressure and the fluid density. A DP meter comprises an obstruction to fluid flow and means for measuring the pressure change caused by the obstruction, giving associated flow rate equations for either volume flow rate or mass flow rate wherein these respective flow rate equations are both functions of density. The obstruction is defined by a “primary element” which can be either a constriction formed in the conduit or a structure inserted into the conduit. The primary element can be for example a Venturi constriction, an orifice plate, a cone shaped element or other form.
Pressure tappings are inserted up stream from the primary element and at or in the vicinity of the primary element and the pressure difference between the two points is monitored. The primary element causes a drop in pressure, and it is customary to measure the pressure at the point of the conduit that corresponds to the lowest pressure. For a Venturi meter, this measurement point will correspond to the “throat” of the primary element, that is, the point of the element that has the minimum cross sectional area. (If the precise position of the lowest pressure is not known for a particular geometry of primary element (as for example in the case of Orifice Plate Meters) it is customary to select a stated position where it is known the pressure will be significantly lower than the pressure tapping up stream of the primary element.)
A further example of a type of meter that is suitable for obtaining a mass or a volume flow rate as a function of density is the so called target meter. In its basic sense, a target meters measures the drag force applied to an object inserted into the fluid on a supporting strut. The drag force is a direct function of the flow's dynamic pressure so when the drag and the mass continuity equations are combined the result is that a mass or volume flow rate can be expressed as a function of the measured drag force and the fluid density.
Another type of flow meter is a velocity flow meter, which in its broadest sense can be taken to be any meter that has means to estimate the volume flow rate of a fluid flowing through a conduit. Velocity flow meters are also sometimes called “linear” or “volume” flow meters, which are hereby taken to be equivalent expressions. These flow meters are not dependent on density—they give no mass flow or density output. These meter designs directly estimate the volume flow rate of the fluid regardless of the fluid density. Examples of volume flow meters include Vortex meters, Positive Displacement (P D) meters, Ultrasonic meters and Turbine meters. With the exception of the P D meter these mentioned volume meters are sometimes described as velocity meters as they give the average velocity of the fluid which in turn can be expressed as a volume flow rate as the cross sectional area of the pipe is known. (The P D meter measures the volume flow rate directly).
Currently, mass flow can only be measured directly by a Coriolis meter. Coriolis meters arc, relative to the pipe size for which they arc used, large, heavy, and expensive pieces of equipment. The size problem makes them impractical for use at line sizes above 12″. Also, for practical applications (oil rig, refinery, pharmaceutical plants etc) there are pumps and valves and other components that cause vibrations in the pipe that can sometimes interfere with the frequency readings read by a Coriolis meter. This noise needs to be filtered out of the Coriolis meter readings, but sometimes (depending on frequency ranges) the noise cannot be filtered out as it's too close to the meter's frequency and the reading is drowned out. Therefore, Coriolis meters are not ideal mass meters but they are all industry has that measures mass flow directly. A Coriolis meter also indicates the density of the flowing fluid and through this, one can estimate the volume flow rate.
When a Coriolis meter is not chosen to measure mass flow and density, the density of the fluid has to be calculated directly. This is done by pressure and temperature measurements and for the properties of the known fluid a “P VT” (i.e. Pressure, Volume and Temperature) calculation is carried out to predict the density. This relies on the knowledge of the fluid composition and fluid properties being accurate. As this can change periodically in flows such as oil and natural gas production this is not ideal as the density measurement is therefore not in real time but based on periodic spot checks on fluid composition. If the fluid composition changes it is not discovered that the fluid density prediction is wrong and hence the meters mass flow rate prediction is wrong until the next sample is taken and analyzed.
As an example (it is to be understood that the scope and application of the following invention is not limited to any particular industry), in natural gas flows, a gas chromatograph takes a sample and analyses the gas components. It then feeds that information to a P VT calculation that predicts the gas density from the measured pressure and temperature from the meter. Carrying out this process with the best known equipment in the shortest possible time is said to take around six minutes to produce a gas density. In many cases it takes longer than six minutes. When that is done it automatically starts again. This is the density the flow computer uses to calculate the mass flow rates from the installed meters until it is up dated every gas chromatograph/P VT cycle. If there arc fluctuations in the gas composition (and hence density) at a frequency greater than the frequency of the P VT up date this system does not see it very clearly. The calculation is a static reading of that particular grabbed fluid sample and the system is assuming that is representative of the flow over a period of time. This inaccuracy can be significant. A monitor that gives more up to date predictions of density could be very valuable to industry.
When the density is calculated it can then be used in conjunction with a velocity meter to predict the mass flow rate. Alternatively, the density can be substituted into the DP meter equation for mass and the DP meter equation for volume to give mass and volume flow rates. However, industry does not have a way of checking the density prediction of the P VT calculation and also has no way of measuring mass flow without using a P VT calculation or a Coriolis meter.